Minor error condition handling tweaks for svn2rss.py.
Minor error condition handling tweaks for svn2rss.py.

* contrib/hook-scripts/svn2rss.py
  (__init__): Clean up some error handling a bit, tweaking error
    message strings to have trailing punctuation, and exiting when a
    required module isn't found.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion